The vascular endothelial cell plays a very important role in both accelerated and chronic rejection. Current assays, particularly the lymphocytotoxicity test, have not uniformly predicted accelerated rejection or correlated well with post-transplant renal function. During the past three years we have shown; a) the indirect fluorescent antibody assay is a reliable method for detecting anti-vascular endothelial antibody as IgG, and b) antibody (IgG) to the vascular endothelial cell is found in high incidence in patients with; 1) accelerated rejection, 2) totally rejected renal allografts, and 3) poor renal function following transplantation. The presence of anti-vascular endothelial antibody is a more sensitive indicator of sensitization than the lymphocytotoxicity test and single cell suspensions of vascular endothelial cells are a more sensitive target than whole vessels. Our current proposal aims to further define the clinical role of this antibody and to characterize its immunological properties and specificities. In particular, we wish to distinguish it, if possible, from anti-HL-A antibodies and to determine if it is cell-specific. Attempts will be made to produce an antisera specific for the antigen in the vascular endothelial cells and the feasibility of using frozen panels of vascular endothelial cells as target antigen will be studied.